Condoms and gloves are typically made from vulcanized natural rubber. Natural rubber is produced in latex form by the Hevea brasiliensis tree and has unique characteristics. These characteristics make natural rubber particularly useful for the preparation of barrier protection products. Among the unique characteristics of natural rubber is its high level of stereo-regularity, meaning that the polymer of which it is comprised is a chain consisting almost exclusively of cis-1,4 isoprene units. Natural rubber latex is also a highly branched polymer with a high molecular weight and a wide molecular weight distribution. These characteristics of the base latex result in vulcanized rubber film products having a unique combination of strength and elasticity. However, natural polyisoprene also contains proteins that have been shown to produce dermal allergic reaction in some susceptible individuals.
Synthetic polyisoprene has been developed to provide a material with the benefits of natural rubber and to eliminate the potential for protein allergy. However, development of a true replacement for natural rubber has proved to be difficult, with synthetic variants such as that produced by Kraton Inc. by anionic addition polymerization typically has a lower level of stereo-regularity (i.e., less than 90% cis 1,4 isoprene) and reduced molecular weight characteristics. This molecular character, in turn, has resulted in synthetic polyisoprene films having an inferior balance of properties compared to those of vulcanized natural rubber films. Consequently, the addition of a cross-linking agent tends to produce more inter-particle cross-links and less intra-particle cross-links during post-vulcanization, resulting in nonhomogeneous cure properties leading to latex film articles having poor strength and elongation properties, such as voids and cracks due to the formation of fractures in the inter-particle regions. In addition, synthetic polyisoprene latex flocks more easily, which result in defects in dipped films, and the latex dip tank has a very limited lifetime that is available for dipping articles. It is, therefore, imperative that synthetic polyisoprene films are cross-linked better to mimic the branched polymeric structure of a natural rubber, thereby providing improved properties.
In dip molding processes, the majority of work with synthetic or natural polyisoprene has been focused on the development of polyisoprene gloves, using a coagulation dip process. In this type of process, a glove-shaped mold is first dipped into a coagulant solution that is known to destabilize the latex formulation. The resulting coagulant layer is then dried, before the mold is dipped into a bath of a compounded latex formulation to form a coagulated wet latex gel. This coagulated wet latex gel is typically leached in water to remove residual surfactant before being dried at a relatively high temperature to complete the cross-linking of the rubber film. The use of a coagulant layer is undesirable in the manufacture of condoms because it impedes the ability to produce a thin latex layer and therefore condoms are dipped over a coagulant free former.
The use of vulcanizing or sulfur cross-linking agents in the manufacture of rubber articles is well-known. The effectiveness of sulfur crosslinking agents is improved by conventional accelerators including dithiocarbamates, thiazoles, guanidines, thioureas, amines, disulfides, thiurams, xanthates and sulfonamides. The use of vulcanizing agents in the manufacture of polyisoprene rubber is disclosed in D'Sidocky et al., U.S. Pat. No. 5,744,552, and Rauchfuss et al., U.S. Pat. No. 6,114,469.
U.S. Pat. No. 3,971,746 to Hirai et al. discloses synthetic polyisoprene rubber latex produced by emulsifying a solution of polyisoprene rubber in an organic solvent including 4-20 wt % of benzene, toluene or xylene with water. After dipping, the solvent is removed by evaporation from the resulting oil-in-water emulsion.
U.S. Pat. No. 4,695,609 to Stevenson discloses vulcanizable rubber compositions containing less than 0.4 parts by weight of nitrosatable materials per 100 parts by weight rubber of xanthogen polysulfide and xanthate compounds. This rubber composition contains a dihydrocarbyl xanthogen polysulphide and a xanthate selected from metal hydrocarbylxanthates and dihydrocarbylxanthates. While commercial aqueous latex compositions are discussed in Examples 9A-E, the aqueous latex compositions do not comprise synthetic polyisoprene. Furthermore, the aqueous latex emulsion 9E contains sulfur, zinc oxide and zinc diethyldithiocarbamate, is stable for only four days, and is capable of producing a product having a tensile strength at fracture of only 22.4 MPa, and an elongation of 830%.
U.S. Pat. No. 5,254,635 to Stevenson discloses a rubber composition containing dibenzylthiuram sulfide. A dibenzylthiuram sulfide, such as tetrabenzylthiuram disulphide, is combined with a dihydrocarbyl xanthogen polysulphide and/or a xanthate to provide a composition, which cross-links natural rubber at 120-180° C. without providing harmful nitrosatables. This natural latex composition, however, is sulfur-free and does not cross-link intra particle regions of a synthetic cis-1,4-polyisoprene having low levels of stereo-regularity. Therefore, the use of this cross-linking agent package for synthetic polyisoprene latex will result in a non-uniform article with inferior properties.
U.S. Pat. No. 6,221,447 to Munn et al. discloses the preparation of hypo-allergenic rubber products, which shrink from a second shape and size to their original shape and size on application of heat. The examples include a polyisoprene condom, which will shrink to fit the individual user during use. The curing package used to make this condom consists of agents such as peroxides and/or sulfur.
U.S. Pat. No. 6,391,326 to Crepeau et al. discloses stable emulsions, methods of preparation, and applications, such as in the formation of elastomeric films. The stable emulsions for preparing an elastomeric film comprise (1) a phase A containing an elastomer dissolved in an organic apolar or slightly polar solvent, in which is dispersed (2) a phase B containing a polymer in solution or dispersed in a polar solvent, which is immiscible with phase A, and (3) a dispersing agent selected from the group consisting of block and grafted polymers. Droplets of phase B having a diameter of 10μ form in phase A. Crepeau et al. does not teach or suggest methods of stabilizing a synthetic polyisoprene latex emulsion against ‘flock’ formation.
U.S. Pat. No. 6,618,861 to Saks, et al. discloses medical gloves with watch viewing capabilities. This patent discloses a polyisoprene latex compound that includes an accelerator system of 2.0 parts per hundred (“phr”) tetramethylthiuram disulfide (“TMTD”), 0.2 phr zinc 2-mercaptobenzothiazole (“ZMBT”), 0.2 phr zinc dibutyldithiocarbamate (“ZDBC”), 0.2 phr 1,3-diphenyl-2-thiourea and 0.2 phr zinc diethyldithiocarbamate (“ZDEC”). However, after curing, this accelerator system provides a product having a tensile strength only of about 1,900 psi.
U.S. Pat. Nos. 6,653,380 and 7,048,977 to Dzikowicz disclose latex film compound with improved tear resistance. The method of enhances the tear resistance, tensile strength, and the aging properties of a latex product by adding an antioxidant synergist with an antioxidant to a latex compound. The latex compound comprises a polymer, a stabilizing system, a film surface conditioner and a curing system that comprises an activator, crosslinker and accelerator. Antioxidant synergists include 2-mercaptobenzimidazole (MBI), 2-mercaptotoluimidazole (MTI), zinc 2-mercaptobenzimidazole (ZMBI) and zinc 2-mercaptotoluimidazole (ZMTI). The latex products formed may be gloves but can also include threads, balloons and other latex-related products. The latex used is not synthetic polyisoprene and the addition of anti-oxidants does not pre-vulcanize the synthetic polyisoprene latex.
U.S. Pat. No. 6,828,387 to Wang et al. discloses polyisoprene articles and a process for making the same. This process produces synthetic polyisoprene articles exhibiting tensile strength properties similar to those of solvent-based processes using natural rubber latex. The process combines a synthetic latex with sulfur, zinc oxide and an accelerator composition comprising a dithiocarbamate, a thiazole, and a guanidine compound, all three of which need to be present, at the pre-cure stage. In a preferred embodiment, the accelerator composition comprises zinc diethyldithiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT), and diphenyl guanidine (DPG), in conjunction with a stabilizer, which is primarily milk protein salt, such as sodium caseinate. Polyisoprene latex (typically 60% solids) and the stabilizer (e.g., sodium caseinate) are combined at ambient temperature (about 20-25° C.). After mixing for a period of time, the mixture is then diluted to 40% solids in water. Wingstay L is then added, and the mixture is stirred for approximately 15 min. At this point, the pH can be adjusted to a range of about 8.5 to 9.0. Zinc oxide is added, followed by the sulfur and accelerator compounds. The elastomeric polyisoprene product made by the process is a surgeon's glove dipped over a coagulant-coated former. The aqueous latex emulsion is stable with a maximum stability of eight days. The tensile strength of the surgical glove product obtained is approximately 3,000 psi (20.6 MPa) (according to ASTM D412). The accelerators are added to the latex emulsion, but maintained at a low temperature for up to eight days. The dithiocarbamate, a thiazole and a guanidine accelerators must be present in the latex together. The latex stabilizer is sodium casinate. The stability of this aqueous latex composition is better than that of Stevenson (U.S. Pat. No. 4,695,609). The glove formers are dipped in a coagulant solution containing calcium nitrate that is unsuited for coagulant-free dipping of synthetic polyisoprene latex condom.
U.S. Pat. No. 7,041,746 to Dzikowicz discloses accelerator system for synthetic polyisoprene latex. The accelerator system comprises dithiocabamate and thiourea and can produce synthetic polyisoprene films having a tensile strength of about 3,000 psi to about 5,000 psi at low curing temperatures. The accelerator system does not contain tetramethylthiuram disulfide or diphenylguanidine or sodium dibutyldithiocarbamate (SDBC), or diisopropyl xanthogen polysulphide (DXP) but contains thiourea. The accelerators are not indicated to pre-vulcanize the synthetic polyisoprene particles and the latex article produced has a very low modulus of 1.5 MPa at 300% elongation and a tensile strength of 20.6 to 34.4 MPa.
UK patent application GB 2,436,566 to Attrill et al. discloses minimizing pre-vulcanization of polyisoprene latex. This process for making a polyisoprene latex comprises compounding a synthetic polyisoprene latex with compounding ingredients and maturing the latex at a low temperature so as to minimize pre-vulcanization. Dipping of condoms is also conducted at low temperatures typically 15° C. to less than 20° C. The absence of pre-vulcanization is verified be assuring the strength of a ring made has a prevulcanisate relaxed modulus has a value less than 0.1 MPa indicative of the absence of pre-vulcanization. The latex emulsion may contain accelerator such as dithiocarbamate. The '566 patent application teaches away from pre-vulcanization prior to dipping of latex articles.
There is a need, therefore, for a stable synthetic polyisoprene latex emulsion composition that does not agglomerate or flock, providing usable emulsion lifetimes. The composition should achieve substantial intra-particle and inter-particle crosslinking in the final product. Such a composition would enable the dip-forming of articles in the absence of a coagulant, such that articles having thinner, continuous, and defect-free layers with enhanced strength and improved stretchability could be obtained. Such articles would not deteriorate and would maintain their physical integrity over time. It is an object of the present invention to provide such a composition, as well as a method of preparing and using such a composition to dip-form articles, and the articles so produced. These and other objects and advantages, as well as additional inventive features, will become apparent from the detailed description provided herein.